Various gels having a three-dimensional network have been designed and developed to meet specific physicochemical properties, particularly in terms of reversible volume change in response to stimuli, e.g., pH, temperature and light, so that they can be applied in various fields such as drug delivery, shape memory element and chemical sensor (Kataoka, K., et al., J. Am. Chem. Soc., 1998, 120, 12694; Miyata, T., et al., Nature, 1999, 399, 766; Hu, Z., et al, Science, 1995, 269, 525; and Holtz, J. H., et al., Nature, 1997, 389, 829).
For example, a hydrogel comprised of poly-(N-isopropylacrylamide) substituted with a phenyl boronic group as its backbone changes its volume depending on the concentration of glucose (Kataoka, et al., J. Am. Chem. Soc., 1998, 120, 12694). Further, a hydrogel which changes its volume on contact with an antigen is used as a drug delivery vehicle (Miyata T. et al, Mature, 1999, 399, 766).
However, most of the conventional hydrogels are based on high molecular weight polymers which are not satisfactory in terms of drug delivery because of their limited bioavailability.